The principal objective of this proposed research is the study of a reaction sequence which serves as a new colorimetric method for carbon monoxide in blood and other tissues. The method appears to be selective for carbon monoxide. The response is linear and the precision of measurements is good. The sequence of reactions takes place in approximately neutral solution and produces a soluble colored compound for spectrophotometric determination. Carbon monoxide initially reduces a palladium(II) compound which in turn reduces an iron(III) complex such as ethylenediaminetetra-acetatoferrate(III) to the iron(II) complex. A free complexing agent such as 1,10-phenanthroline is present to competitively complex the iron(II) and form a colored chelate. Sodium molybdate is used as a catalyst. Studies to improve the method and make it suitable for routine manual and automated analysis will include the nature of the carbon monoxide-palladium(II) reaction, the selection of complexing ligands for iron(III) and iron(II) to facilitate the ligand exchange reaction, and a continuing search for more effective catalysts.